1. Field of the Invention
The present invention relates to a method of high precision position measurement of two-dimensional structures such as measuring structures on wafers or masks, and also relates to a reference mask for use in connection with the method.
2. Description of the Prior Art
In the manufacture of integrated semiconductor circuits, the build-up of the components and structures proceeds from one surface of a semiconductor wafer composed of a silicon crystal and referred to as the wafer. The silicon crystal serves both as a carrier for the overall circuit as well as the semiconductor material for the active components such as transistors and the like. The different dopings required in the semiconductor are generated from the surface of the semiconductor wafer either by means of diffusion technology or by means of ion implantation technology. Usually, a plurality of doping steps is sequentially necessary in various structures. The connecting and terminal lines are then produced, being laid on oxide layers provided with windows, and usually being laid one above the other at a plurality of structure levels. The transfer of the successively manufactured structures onto the semiconductor wafer occurs by means of a photolithographic process, in which the semiconductor wafer is coated with a photo-resist at every structuring stage and this photo-resist is then exposed through a mask having the corresponding desired structure and is then developed. With increasingly smaller structured dimensions and increasingly larger wafers, the congruence or coincidence of the structures to be successively generated on the wafer in various photo-technique levels becomes increasingly important. The causes of registration errors of the structures to be successively transferred by photolithography onto the semiconductor wafer occur in the adjustment of successive masks relative to the wafer as well as in the scattering of the positions of the individual structure elements in the wafer and in the mask. Such scattering can be attributed to systematic errors of the exposure apparatus employed, to a warping of the wafer or mask in the progress of the chemical processes, or other causes which lead to a warping of the mask or wafer.
German Pat. No. 2 822 269 discloses a method for automatic mask adjustment wherein the adjustment of successive masks is undertaken with adjustment structures proceeding orthogonally relative to one another on the wafer coated with a photo-resist and on the mask. The adjustment structure of the wafer is produced in the first structuring process on the surface of the wafer, for example, by etching, so that the adjustment structures present on the corresponding mask in all following structuring processes can be aligned relative to this first adjustment structure. For automatic mask adjustment, the adjustment structures on mask and wafer are then opto-electronically scanned in directions parallel to their edges with the aid of an image sensor so that the intensity of the picture signal generated by the opto-electronic scanning is integrated on a line-by-line basis or in line sections and the analog integral values resulting are converted into digital gray scales and are stored. By forming the difference of the gray scales of successive lines and further evaluation of the resulting different signals, the position of the center axes of the adjustment structures can then be determined. Then, wafer and mask are displaced relative to one another for adjustment in accordance with the identified offset of the center line.
In view of the problem of scattering the positions of the individual structure elements in the wafer or in the mask, absolute position measurements were heretofore performed by means of specific measuring instruments. The subject edges were photo-electrically determined and the spacing was identified by means of laser interferometers by moving the mask or the wafer. This arrangement requires ultra-high precision of the mechanics, a low degree of vibration, and a temperature stable environment as well as a regular calibration of the laser interferometers by corresponding norms. Overall, the installation of such a measuring possibility requires high capital costs, and the measuring reliability obtained over the long run is determined by the quality of the regular calibration.
The article "Metrology in Mask Manufacturing" in IBM J. Res. Develop., Vol. 26, No. 5, September 1982, pages 553 to 560, discloses a method for the identification of registration errors on the structures on the wafer and mask with which the position measurement is undertaken with the use of quadratic measuring structures on the mask and wafer. These quadratic measuring structures are of different sizes on the mask and on the wafer so that the center deviations in squares nested in one another indicate topical registration errors. Absolute position measurements of two-dimensional structures on the mask or wafer, however, are not possible with this method.